Sputtering is a well known method of depositing a film layer onto a semiconductor substrate. A typical sputtering apparatus includes a vacuum chamber that encloses a target and a substrate support pedestal. The target is typically affixed to the top of the chamber, but is electrically isolated from the chamber walls. A voltage source maintains the target at a negative voltage with respect to the electrically grounded walls of the chamber to excite into a plasma state inert gas (typically argon) which is supplied to the chamber and maintained in the chamber at a low pressure. Ions from the plasma bombard the target and eject particles of target material from the target toward the substrate. These particles deposit on the substrate to form the desired film. While the goal of sputter deposition is to create an even deposition of a film layer onto the substrate, the sputtered particles typically deposit on other exposed chamber surfaces. In addition, a portion of the sputtered particles will typically be back sputtered onto (i.e., returned to) the target.
Magnetron sputtering is one method of increasing sputtering, and thus the deposition rate. It employs a magnetic source to create an arched magnetic field superimposed on the electric field created between the target and the grounded elements of the chamber. The magnetic source is generally provided behind the target and commonly comprises inner and outer pole pieces and a yoke connecting the opposite pole's pieces. The magnetic flux generated from this source exits from and returns to the surface of the target, thereby forming an arched magnetic field adjacent to the surface of the target. The arching magnetic field includes both parallel and perpendicular field line components relative to the target surface.
Application of a negative DC bias to a magnetron target results in a sputtering pattern having maximum erosion of the target in the regions where lines of magnetic flux are parallel with the surface of the target, in this case in the region just between the poles of the magnetic source. Where the location of a parallel component of the magnetic field remains constant during the sputtering process, the maximum erosion of the target typically takes the form of a groove in the target corresponding to the region where the magnetic field lines intersect the electric field at a right angle. Typically, such grooved erosion patterns are in the form of an oval track on the target, but depend upon the location of the stationary magnets with respect to the target surface.
If the DC bias on the target is changed to RF, the greatest sputtering will occur where the component of the magnetic field is perpendicular to the target and erode the target in areas in exact relief and complementary to the target erosion pattern created by DC bias.
The life of a target is limited by progress of erosion in the annular areas. When either DC or RF magnetron sputtering is continuously applied, the target has to be replaced frequently because of persistent target consumption in the corresponding annular grooves. As an annular groove progressively forms in the target during sputtering thereof, the slightly or non-eroded regions tend to accumulate back sputtered material because of non-erosion of these regions. Where the back sputtering material does not form a tight bond with a target, the material may flake off and contaminate a substrate. In the groove regions, any back sputtered material is quickly re-sputtered off the target.
To avoid contamination of the work piece, and extend the useful life of the target, attempts have been made to clean the targets, once removed from the sputtering chamber, to prevent particulate contamination of the work pieces being processed on an ongoing basis. Currently, when targets, such as strong magnetic materials targets, are removed from the vacuum of the process chamber, the targets typically flake and prove difficult to clean. Further, the strong cathode magnetic field behind the target makes cleaning of the strong magnetic materials targets even more difficult.